BIOC 460 - DR. TISCHLER - LECTURE 24 DIGESTION & ABSORPTION OF CARBOHYDRATES

OBJECTIVES 1. Principal dietary monosaccharides and disaccharides. 2. General structural features of amylose, amylopectin, glycogen and cellulose. 3. Products of starch digestion by pancreatic α-amylase. Four glycoprotein brush-border enzyme complexes, their roles in digestion of carbohydrate, products of their actions. How the primary nutritional monosaccharides (glucose, fructose, and galactose) are moved from the intestinal lumen to capillaries. 6. Tissue where each facilitative glucose transporters (GLUT 1 to GLUT 5) is found; describe their basic function

Figure 1. Structures of the most common dietary monosaccharides. GLUCOSE CH2OH OH FRUCTOSE HO Figure 1. Structures of the most common dietary monosaccharides.

Figure 1. Structures of the most common dietary disaccharides. CH2OH SUCROSE (glucose - fructose) -1,2-bond OH O MALTOSE (glucose - glucose) CH2OH OH -1,4-bond O LACTOSE (galactose - glucose) OH CH2OH -1,4-bond O TREHALOSE (glucose - glucose) CH2OH OH -1,1-bond Figure 1. Structures of the most common dietary disaccharides.

O -[1- 4] linkages -[1-6] linkage ........ CH2OH CH2 Figure 2. General structure of amylopectin (principal plant polysaccharide) and glycogen (principal animal polysaccharide)

TYPES OF POLYSACCHARIDES Amylose: plant CHO storage linear chain with 1,4 glycosidic bonds Amylopectin: amylose+branches with 1,6 bonds Glycogen animal CHO storage similar to amylopectin except more branches Cellulose: linear chain with 1,4 glycosidic bonds

Figure 3. Digestion of carbohydrates – Top – Early digestion depicting the intake of dietary carbohydrates and the role of salivary amylase

Figure 3. Digestion of carbohydrates Middle – Luminal digestion showing the combined roles of the stomach, pancreas and intestine.

16 e t c . 14 -amylase Maltotriose Figure 4. -Amylase, of salivary or pancreatic origin, cleaves amylopectin to produce all of the products shown

16 e t c . 14 -amylase -limit dextrin (branched oligosaccharide; unbranched also produced) Figure 4. -Amylase, of salivary or pancreatic origin, cleaves amylopectin to produce all of the products shown

16 e t c . 14 -amylase Maltose Glucose Figure 4. -Amylase, of salivary or pancreatic origin, cleaves amylopectin to produce all of the products shown

Figure 3. Digestion of carbohydrates Bottom – Summary of the brush border glycoprotein complexes containing disaccharidases and oligosachharidases.

Lactase/-galactosidase Lactase   Table 1. Brush border complexes of the luminal membrane (brush border). Enzyme Complex Sucrase-isomaltase: Sucrase Maltase Isomaltase Glucoamylase (exo)amylase Lactase/-galactosidase Lactase Trehalase: Trehalase Natural Substrate   Sucrose (table sugar) Maltose; maltotriose -limit dextrins; isomaltose Oligosaccharides; including -limit dextrins; not 1 6 bond Lactose (milk sugar) Trehalose (mushrooms) Product(s) Fructose + glucose Glucose Galactose + glucose

Primary – defective lactase enzyme at birth late onset appears gradually in late teens/adults Secondary (acquired) – consequence of diseases that damage brush border tropical sprue – bacterial infection celiac sprue – gluten-sensitive enteropathy

Normal intestinal villi Patient with celiac sprue

Figure 5. Absorption of monosaccharides Fructose; Glucose Na+ Galactose Lumen of also glucose intestine Galactose Glucose Intestinal Na+ SGLT-1 GLUT-5 Epithelial cell Brush border Galactose Glucose Na+ Fructose (+glucose) Glucose + Galactose Na+ 2K+ GLUT-2 ATP 3Na+ 2K+ contraluminal membrane ADP + Pi 2K+ 3Na+ to capillaries 3Na+ 2K+ = facilitated diffusion = Na,K-ATPase = Na + - dependent co - transport Figure 5. Absorption of monosaccharides

Table 2. Types of Facilitative Glucose Transporters Tissue Function GLUT-1 many mammalian tissues (e.g., red blood cell, brain, kidney) glucose uptake GLUT-2 Liver; pancreas -cell, contraluminal membrane of intestinal cell high capacity; low affinity; regulates insulin release by pancreas; liver: removes glucose after a meal GLUT-3 many mammalian tissues; especially brain, kidney GLUT-4 skeletal muscle, heart, adipose insulin-stimulated glucose uptake; helps prevent hyperglycemia through insulin action GLUT-5 small intestine primarily fructose absorption